1. Field of the Invention
Fixing rollers are used in the fixing sections of copying machines, line printers, facsimile machines, and the like. This invention is directed to a method for manufacturing fixing rollers.
2. Description of the Related Art
The fixing section of a copying machine is shown in FIG. 3 (PRIOR ART). In a widely employed safe and economical fixing method, a copy sheet 33 on which a toner image 32 has been transferred is passed between a heating/fixing roller 30 and a pressure fixing roller 31. When the sheet 33 passes therebetween, heat and pressure applied to the copy sheet cause the toner image to fuse and fix on the copy sheet.
Heating/fixing roller 30 comprises a roller base member or roller core member 34 made of heat resistant material such as plastic, ceramic or metal. The roller core member 34 is coated with a fluorine resin coating 35 several tens of a micrometer thick for ease in peeling the toner from the surface of the core member.
Pressure/fixing roller 31 includes a core member 36. Core member 36 is coated with a coating 37. A Separation pawl 38 is associated with roller 30 and a separation pawl 39 is associated with roller 31. Reference numeral 40 represents a toner fixed image and reference numeral 41 represents a heater for generating the thermal energy required to fix the toner image 32 to the copy sheet 33.
A resin coating is typically formed on the surface of a tubular core member of a fixing roller by a spray coating method or by an electrostatic coating method. This coating is burnt and provides a rough surface. Accordingly, the surface requires polishing, and, occasionally, further burning.
In the known transfer coating method, a transfer coating roller draws fluorine resin dispersion from a resin dispersion bath onto its spherical outer surface, and applies the resin dispersion over the core member. The surface of the fluorine resin coating, which is formed on the core member by the transfer coating roller and is then burned, is uniform and smooth. Therefore, the polishing and the reburning steps are not required for the surface of the resin coating.
However, the transfer coating method has its disadvantages as well. For example, when the fluorine resin dispersion drawn onto the outer surface of the transfer coating roller is separated from the resin dispersion on the outer surface of the core member, the amount of coating of the resin dispersion is increased wherever the resin dispersions are separated from each other. Thus, the thickness of the resin coating is increased. When the transfer coating method is used, difficulties arise in forming the resin coating uniformly over the entire surface of the core member. In some extreme cases, the difference in thickness across the surface of the roller exceeds 30 .mu.m. When a core member having a resin coating with that large of a thickness gradient is used for the fixing roller of a copying machine, a variety of associated operational difficulties may occur. For example, the fixing of the image is poor. Also, the copy sheet tends to wrinkle. Moreover, A color irregularity or stripe appears on the boundary between the thickness-increased portion of the resin coating and the remaining portion.